The transplantation of embryonic inhibitory neurons has recently emerged as a promising avenue for cell-based brain repair. Previously, we and others have shown that transplanted inhibitory neurons restore juvenile plasticity to the circuits of adult visual cortex. In this study, we set out to elucidate the mechanisms for transplant-induced cortical plasticity. We hypothesize that transplanted inhibitory neurons reactivate plasticity by creating a new, disinhibitory microcircuit in recipient adult visual cortex. First, we will map out the effects of transplantation on the inter- and intralaminar balance of excitation and inhibition. Next, we will compare the cell-type specific effects of brief monocular deprivation on visual activity in host cortex. Lastly, we will test whether transplanted neurons make long-range connections with appropriate circuits. To evaluate these hypotheses, this proposal will take advantage of recent advances in optogenetic dissection of inhibitory circuits in brain slice, multiphoton imaging of defined cortical cell types using genetically encoded calcium indicators, chemical-genetic tools for testing the mechanisms of transplant- induced plasticity and viral tracing and whole brain clearing to identify the brain-wide connections onto transplanted cells. If successful, the proposed studies shed light on the mechanisms of transplant-induced cortical reorganization. These studies are also likely to give insight into the normal developmental regulation of cortical plasticity by inhibitory cells.